1. Field of the Invention
This invention relates to apparatuses for treating wafers utilizing the plasmas produced by gas discharges; this invention also relates to methods of cleaning such apparatus.
2. Description of the Related Art
FIG. 1 is a schematic view of a conventional plasma chemical vapor deposition apparatus. The apparatus of the figure comprises the grounded vacuum vessel 1 in which the wafer or the substrate 2 is placed. The high frequency voltage applying main electrode 3, electrically connected to the high frequency voltage source 4, opposes the substrate 2 from above across a discharge space 5. The outer surfaces of the electrode 3 are covered by the grounding shield 6 electrically connected to the vessel 1. Further, limiter electrode 7, electrically connected to the vessel, surrounds the sides of the discharging space in which plasma 5a is formed. A heater 8 is provided to heat the substrate 2 to a predetermined temperature.
The operation of the apparatus of FIG. 1 is as follows. After the substrate 2 is heated to a predetermined temperature by the heater 8, the deposition gas for forming thin films on the substrate 2 is introduced into the vessel 1 through an inlet port (not shown); a carrier gas may also be introduced into the vessel 1, if necessary. When amorphous silicon films are formed on the substrate 2, for example, silane gas (SiH.sub.4) may be used as the material gas for forming the thin films on the substrate 2, and hydrogen gas (H.sub.2) or the argon gas (Ar) as the carrier gas. Further, a high voltage is applied to the main electrode 3 by the voltage source 4 to start a glow gas discharge in the discharge space 5 between the electrode 3 and the substrate 2, thereby generating a plasma therein. The plasma 5a produced therein is that of the deposition gas and the carrier gas introduced into the vessel 1. The molecules of the deposition gas are decomposed by the impacts of the electrons in the plasma 5a, and a desired thin film is formed on the substrate through a series of chemical reactions.
In this process of thin film formation utilizing the plasma 5a produced by the gas discharge, the limiter electrode 7 which is electrically connected to the vessel 1 is kept at the ground voltage. Thus, the discharge space 5 is limited to the regions surrounded by the electrode 3, the substrate 2, and the limiter electrode 7; thus, the plasma 5a is prevented from coming into direct contact with the inner surfaces of the walls of the vessel 1.
The conventional apparatus as described above, however, has the following disadvantages.
The limitation of the discharge space 5 by the limiter electrode 7 cannot prevent the diffusion of the particles formed by the decomposition of the deposition gas in the plasma 5a. Thus, these decomposed materials are diffused over the whole space in the vessel 1, thereby forming films of the material on the surfaces of the vessel 1 which are situated outside the discharging space 5 contained by the limiter electrode 7; although the speed of the accumulation of the film on the surfaces outside of the discharge space 5 is slow compared with that of the deposition of the film on the substrate 2, the films accumulated on the surfaces of the vessel 1 may peel therefrom and drop onto the surface of the substrate 2 which are undergoing the film formation process, which results in defects in the films on the substrate 2.
The limitation of the discharge space 5 by the limiter electrode 7 reduces the percentage of the volume occupied by the plasma 5a in the whole volume of the vessel 1. This is disadvantageous for the purpose of reducing the amount of impurities such as oxygen (O) and carbon (C) which remain near the surface regions of the walls of the vessel 1 and which have adverse effects on the quality of the film formed on the substrate 2.